For the construction of ships, for example ships' hulls and hold covers, bridges, roofs or multistorey buildings, it is necessary to use structural parts which are capable of withstanding considerable loads produced by external forces. Because of these requirements, such structural parts usually consist of metal plates or metal supports which are strengthened by an appropriate geometry or suitable braces. Because of high safety standards, hulls of tankers therefore usually consist of an inner and an outer hull, each hull being composed of 15 mm thick steel plates which are connected to one another by about 2 m long steel braces. Since these steel plates are subject to considerable forces, both the outer and the inner steel hull are stiffened by welded-on reinforcing elements. Both the considerable amounts of steel required and the time-consuming and labor-intensive production are disadvantages of these traditional structural parts. Moreover, such structural parts have a considerable weight, resulting in a lower tonnage of the ships and an increased fuel requirement. In addition, such traditional structural elements based on steel require a very great deal of maintenance since both the outer surface and the surfaces of the steel parts between the outer and inner hull have to be regularly protected from corrosion.
SPS (sandwich plate system) elements which comprise a composite of metal and plastic are known as a substitute for the steel structure. Adhesion of the plastic to the two metal layers gives composite elements having extraordinary advantages compared with known steel structures. Such SPS elements are disclosed in U.S. Pat. No. 6,050,208, U.S. Pat. No. 5,778,813, DE-A 198 25 083, DE-A 198 25 085, DE-A 198 25 084, DE-A 198 25 087 and DE-A 198 35 727. Usually, these composite elements are produced by pouring or injecting the starting materials for the preparation of the polyisocyanate polyadducts in a single operation between the metal plates. Since the reactive starting components for the preparation of the plastics in the composite element begin to react as early as during mixing and complete filling of the space between the metal plates is essential for a satisfactory product, the process of injection of the starting components is a decisive and critical step in the production of the composite elements.